1. Field of the Invention
The present invention relates to a cross-linked foam and a manufacturing method thereof. More specifically, it relates to a cross-linked foam having various inner cavity structures formed by an internally-formed surface and a method and device for forming the inner cavity structure simultaneously with a body of the cross-linked foam.
2. Discussion of the Related Art
FIG. 30 is a flow chart illustrating process steps for manufacturing cross-linked foams according to a related art. In step S10, source materials including main material such as diverse resins and other additives are first weighed in accordance with a designed mixture standard depending on what kind of cross-linked foam is fabricated. Then the weighed resins and additives are mixed with a cross-liking agent and a foaming agent in a hermetical mixer or kneader in a milling process. Therefore, a mixed chemical compound is prepared.
In step S20, the prepared chemical compound is provided into a calendar roll or an extruding machine. The calender roll transforms the chemical compound in a form of sheet or film, e.g., a two dimensional shape, and the extruding machine transforms the chemical compound in a form of pellet, e.g., a three dimensional shape.
Step S30 shows various process steps for forming a desired cross-linked foam. The process of forming the cross-linked foam may be classified into a pressure cross-linked foaming method (pressure cross-linked foam molding) and a normal pressure cross-linked foaming method depending on machinery and equipment for the processes considering shapes and properties of the desired cross-linked foam.
The pressure cross-linked foaming method mainly uses a metallic mold(s) to make the desired cross-linked foam, and applies heat and pressure to the chemical compounds after an input of the chemical compound into the inner parts of the metallic mold(s). Therefore, the cross-linked foam having a discontinuous pattern is formed in accordance with an inner part shape of the metallic mold(s) by a decomposition action of the foaming agent. Such pressure cross-linked foaming method may includes, for example, a compression-press cross-linked foam molding method that uses a press machine, and a injection-press cross-linked foam molding method that uses an injection machine, as shown in the step S30 of FIG. 30.
When using the compression-press cross-linked foam molding method, the source materials are first put into the opened mold, and then the mold including the source material is closed. When using the injection-press cross-linked foam molding method, the source materials are put into the airtight injection mold. However, both in the compression-press and injection-press cross-linked foam molding methods, once the source material is provided into the mold, equipment such as press machine applies heat and pressure to the closed mold to foam the source material into a cross-linked foam
In step S40, the applied pressure is released, and then the closed mold is open to de-mold the cross-linked foam. The de-molded material is then cured for a time period and cooled down to a desired temperature. In step S50, the cured and cooled cross-linked foam is then cut and trimmed to be a final product.
Although not shown in FIG. 30, the pressure cross-linked foaming method may also include a compression-rotary press cross-linked foam molding method where heating rolls and metallic press/conveyer belts are used to apply heat and pressure to the source materials for the cross-linked foam by way of inserting the source materials between the heating rolls and the metallic press/conveyer belts. Alternatively, the compression-rotary press cross-linked foam molding method may insert the source materials continuously with other textile materials or rubbery materials between the heating rolls and the metallic press/conveyer belts, whereas the foaming of the source material is induced at a point where the pressure is discharged. Thus, the cross-linked foam may have continuous and uniform surface and cross section.
The pressure cross-linked foaming method applies heat and pressure directly to the source materials using the metallic moulds and rolls. The compression-press cross-linked foam molding method produces various large or small industrial foams, for example, EVA, PE, rubbery large or small sponge panels, shoe components, sports goods and accessories, and the like. The injection-press cross-linked foam molding method generally produces various industrial foams having individual shape, for example, EVA-based shoe components, sports protectors and goods, bags, accessories and the like. The compression-rotary press cross-linked foam molding method produces various industrial continuous roll types or large panel type foams, for example, EVA, PE or other rubbery continuous rolls.
Meanwhile, the normal pressure cross-linked foaming method is widely used for forming a cross-linked foam having a continuous and uniform cross section. Unlike the pressure cross-linked forming method, the normal pressure cross-linked foaming method produces the cross-linked foams without a direct heat and pressure infliction on the source materials. The normal pressure cross-linked foaming method is classified into a chemically cross-linked foaming method and an electron irradiation cross-linked foaming method.
The chemically cross-linked foaming method adds and mixes a chemical cross-linking agent, a foaming agent, and an EVA based resin into a polyethylene resin that is a main source material. Thereafter, the mixture is extruded into a pellet type foaming material as shown in step S10 and S20 of FIG. 30. Then, through the step S30 of FIG. 30, the foaming materials are inserted into a hopper of the extruding machine that includes screws, heat appliers and extruding dies, and then the heat pre-determined by the material composition is applied to the foaming materials. After that, the melted foaming material passes through the extruding dies to provide a continuous and uniform cross section, and then the foaming material is foamed thereby (in step S30).
The electron irradiation cross-linked foaming method applies electron rays to a foaming material that is formed by extruding a polyethylene or polypropylene resin mixed with other additives and agents, thereby cross-linking the materials and heating the foaming material up to the foaming-agent's decomposing temperature to make the foams. This electron irradiation cross-linked foaming method differs from the chemically cross-linked foaming method in a way that this uses the electron rays to achieve the cross-linking and then heats the cross-linked foaming material to foam the cross-linked foaming material.
Meanwhile, in step S40, the cross-linked foam is cured for a time period and then cooled down to a certain temperature. In step S50, the cross-linked foam is then cut, trimmed and designed to be a final product.
Although not shown, the normal pressure cross-linked foaming method includes a calender cross-linked foaming method in which a mixture of a polyvinyl chloride based or polyolefin based resin with a chemical foaming agent, a cross-linking agent (plasticizer in case of polyvinyl chloride), a stabilizer and a surfactant is used. The calender cross-linked foaming method transforms the mixture into a continuous and uniform foaming material using the extruding machine, the storage mill and the calender roll, and then the foaming material is heated in the heating chamber of a conveyor to be foamed under a normal pressure condition. Thereafter, the foaming material is cooled down and cured for a time period to form the foams, and then a roll-shaped foam is obtained by way of winding the foams on the take-up roll.
In step S60 of FIG. 30, the foams finally obtained through the pressure cross-linked foaming method or normal pressure cross-linked foaming method may be attached to one of other molded materials formed with a material that is the same as or different from the foaming material, textiles, woods and metallic materials depending on an end use, property and purpose of the foams and then be re-formed.
Such a re-forming method may be classified into a heat/cold mold compression re-molding, a cold mold compression re-molding, a cold mold vacuum re-molding, and a blow re-molding. The heat/cold mold compression re-molding method forces the cross-linked foam to be inserted into the mold, and then the cross-linked foam in the mold is cooled down after being heated and pressed. The cold mold compression re-molding method pre-heats the cross-linked foam and then inserts it into the mold, and thereafter, the cross-linked foam is pressed and cooled down to form the finalized foam. The cold mold vacuum re-molding method applies heat to the cross-linked foam at a pre-determined temperature and then sucks the heated foam into the mold using a vacuum pressure, and thereafter, the foam is cooled down and de-molded to form the finalized foam. Furthermore, the blow re-molding method applies heat to the cross-linked foam to be softened and then inserts the high-pressure gas or the liquid into the cross-linked foam, such that the cross-linked foam is re-molded in the mold and becomes the finalized foam after being de-molded.
The cross-linked foams formed by the related art cross-linked foam fabrication methods have the following characteristics. The compression-press cross-linked foam molding method of the pressure cross-linked foaming method inserts the source material shaped like a sheet type or a particle type into the mold, and then applies heat to that source material, thereby manufacturing the foam having a uniform physical property. Furthermore, since the injection-press cross-linked foam molding method heats the source material in the cylinder of the feeder and then melts the material so as to be inserted into the mold, the source material can have the uniform property in all parts and the produced cross-linked foams may also have the uniform physical characteristics.
Meanwhile, since the normal pressure cross-linked foaming method inserts the source materials having a particle type into the extruding machine and then heats them to be softened, the cross-linked foam material can have the uniform cross section and the uniform physical property in all parts, and also the finalized foam may have the uniform property in all portions. Although the source material is formed of the several substances in the related art cross-linked foam fabrication, the finalized foam also has the determined property having the uniform density because the source material is transformed into a single unified material before the foam process. Moreover, in the related art methods described above, the foaming process does not make the foam having different density or different properties in every each portion because the same source material is used in the foam process. The related art cross-linked foaming method is hard to manufacture an inner cavity structure having various shapes and formations inside the foam at the same time when the foam is made. Therefore, the related art cross-linked foaming method does not make the density differentiation inside the finalized foam.
Therefore, when manufacturing the cross-linked foam having the complex physical properties and functions, the related art separately makes the cross-linked foams and then cuts, grinds and attaches the foams in additional fabrication steps to produce the foam having the diverse densities and desired inner structures. However, such additional processes may cause the fabrication difficulty, the low throughput and the degradation of design and quality, such that the desired cross-linked foam having the various physical properties and inner structures is hardly obtained. Moreover, the related art described hereinbefore may increase the process steps and costs and may cause industrial wastes because the foams each having different physical properties and functions are separately manufactured and compounded.
To overcome the above-mentioned disadvantages, the Korean Patent Application No. 2003-45282 titled “Method for Manufacturing EVA Based Foam” has disclosed a method including steps of 1) mixing an EVA resin, a cross-linking agent, a foaming agent, a colorant, a filler, an additive, and a rubber or a resin which can be mixed with the EVA resin, 2) performing a low melting point spinning on the resultant composition, 3) making the spinning filament into a tow or staple fiber to be used as a first material, selecting a second material from a group consisting of a water soluble PVA based staple fiber, a polyester based staple fiber and a natural fiber, and producing a non-woven fabric by mixing the first and second materials, 4) melting out a dissolved matter from the non-woven fabric, and 5) cross-linked foaming the non-woven fabric. This method has merits in that an air pore structure is formed in the foam. However, the method disclosed in the above-described Korean Patent Application No. 2003-45282 is not concerned with a method for designing or controlling the shape and structure of the inner surface shape and structure of the foam, whereby the cross-linked foam could not have the different densities and functions in the parts.